The present invention relates to electric power plants and more particularly to ID fan controls employed therein.
The boiler in an electric power plant usually includes one or more forced draft (FD) fans which drive air into the boiler for combustion purposes and one or more induced draft (ID) fans which draw combustion products out of the boiler for cleanup and discharge. It is generally desirable to coordinate the control of the FD and ID fans so that the internal furnace pressure is slightly negative to avoid outflow of combustion products directly to the atmosphere through small furance openings which may exist. The furnace normally has sufficient structural strength to withstand suction pressures substantially greater than that associated with normal furnace operation.
During boiler startup, furnace suction pressure excursions as high as 30 to 50 inches water can occur quickly, particularly in view of the lower temperature and increased density of the air during startup. The magnitude of the suction pressure excursion can be so great that a costly and hazardous furnace implosion would occur. The possibility of an implosion is greater with the larger more recent boiler and ID fans because the destructive fan suction force increases significantly with larger ID fans. Implosion possibilities also are greater in cases where ID fans are retrofitted to older boilers which typically have less structural strength than do the more modern boilers. It is therefore desirable that an ID fan control be provided to limit suction pressure excursions during startup in a safe and reliable manner.